Godzilla's (in)famous "Victory Dance"◊ after he and Rodan team up to kick King Ghidorah's ass in their first showdown on Planet X. As cheesy as it is, the Big G earned it. And there are still plenty of Zillaphiles who can't help but grin at that scene.
The first battle on Planet X, in and of itself. Sure, we've seen three-way monster brawls in this series before...but how about one IN SPACE? Sure, the monster effects are a little cheesy, and the Planet X soundstage isn't likely to fool anyone over the age of 7, but the whole premise of that battle is just Made of Win.
Though not quite on the scale of the climaxes of Ghidorah, the Three-Headed Monster or Destroy All Monsters, the final battle in this film is arguably one of the best in the Showa series. It starts out as a three-way smackdown between all three monsters, the Japanese military, and a whole fleet of Xilien flying saucers...only to blend seamlessly into a two-on-one brawl between Godzilla, Rodan and King Ghidorah once the Xiliens get their asses kicked and flee the planet. Few other Kaiju battles in cinema history feel as epic as that one.